Well drilling is a process used in penetrating subterranean zones (also known as subterranean formations) that produce oil and gas. In well drilling, a well bore is drilled while a drilling fluid (also known as a drilling mud) is circulated through the well bore. The drilling fluid is typically an oil-based fluid comprising oils such as diesel, mineral oil, unsaturated olefins, and organic esters. After drilling the well bore to a desired depth, a string of pipe, e.g., casing, is usually run in the well bore. The drilling fluid in the well bore may be conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set into a hard mass to thereby attach the string of pipe to the walls of the well bore and seal the annulus.
During the drilling process, the drill bit generates drill cuttings as it forms the well bore. Drill cuttings consist of small pieces of shale and rock. The drill cuttings are carried in a return flow stream of the drilling fluid back to the well drilling platform. They are then separated from the bulk of the drilling fluid via conventional separators such as shale shakers, mud cleaners, and centrifuges. Some shale shakers filter coarse material from the drilling fluid while other shale shakers remove finer particles from the drilling fluid. After removing the drill cuttings therefrom, the drilling fluid may be re-used in the drilling process.
The drill cuttings separated from the bulk drilling fluid typically are discharged from the drilling platform to the surrounding area. Drilling platforms are often located offshore in hundreds of feet of water filled with marine life. The drill cuttings thus accumulate in the seabed near the base of the platform. Unfortunately, the drill cuttings may be contaminated with the oil contained in the drilling fluid. This oil must be removed from the drill cuttings before their disposal to meet regulatory limitations. Otherwise, the oil would pollute the surrounding environment and would be particularly hazardous to marine life.
In addition, the crude oil recovered from the subterranean formations often contains sand that must be separated from the oil. Like the drill cuttings, the sand is disposed of by dumping it from the drilling platform into the seabed where it forms sand piles. The sand also may be undesirably coated with the produced crude oil. Thus, the sand could adversely affect the marine environment unless the oil is removed therefrom.
Various methods have been used to remove oil from drill cuttings and sand, thereby meeting certain regulations designed to protect the environment from oil pollution. In one method, the oil is extracted using solvents such as toluene or methylene chloride. However, the potential hazards caused by the toxic nature of the solvents have raised doubts about this method. Another method involves transporting the drill cuttings and the sand onshore and subjecting them to a thermal process. Using such a thermal process can be very expensive, particularly since it is necessary to transport the drill cuttings and the sand to an onshore location.
An improved method has been developed to separate the oil from solid material removed from a wellbore, such as drilling cuttings and sand. In this method, the solid material is passed from the well bore to a separation zone located on or near the drilling platform, thus avoiding the high costs associated with transporting the solid material onshore. Chitosan, water, and a halogenating agent such as bleach are introduced to the separation zone containing the solid material, leading to the formation of a flocculated solid that surrounds oil droplets. As a result, the oil becomes trapped in the flocculated solid such that it is no longer disposed on the solid material. The solid material can then be separated from the flocculated solid and discharged from the drilling platform without being concerned that the surrounding environment could be harmed.
To avoid contaminating the environment with the oil contained in the flocculated solid, the flocculated solid can be collected and properly disposed. It may be transported to an onshore location and incinerated at relatively high temperatures. The costs of transporting the flocculated solid and incinerating it and the oil bound therein can be relatively high. Thus, it is desirable to separate the oil from the flocculated solid such that the oil may be recovered, thus avoiding the problems associated with its disposal. A need therefore exists for an environmentally friendly, economical, and simple method of recovering the oil from the flocculated solid.